Gene Discovery ? Immunotherapy has been most successful when targeted to differentiation antigens expressed on hematopoietic tumors, in large part because the target antigens (e.g., CD20, CD25) are not expressed on essential cells or organs. Other cancers in which this approach could be successful are prostate, breast and ovary. To identify target antigens in prostate we have developed a computer based method to search the EST data base for ESTs that are expressed in prostate, prostate cancer and not other essential organs or cells. We have validated this approach experimentally and used it to identify prostate specific members of the MAGE/GAGE family which we term PAGE. We also found evidence for T-cell receptor gamma transcripts in prostate epithelial cells. This method promises to be a successful approach to the discovery of new targets in immunotherapy.